Semiconductor memory device are typically classified into volatile memory devices and nonvolatile memory devices. Volatile memory devices lose their stored data when their power supplies are interrupted, while nonvolatile memory devices retain their stored data even when their power supplies are interrupted.
Volatile memory devices include static random access memories (SRAMs) and dynamic random access memories (DRAMs), which are roughly categorized according to data storage schemes. That is, an SRAM stores data by using a latch while a DRAM stores data by using a capacitor. Especially, an SRAM is mainly used as a cache memory because its peripheral circuit is simple in configuration and its speed is high in spite of lower memory capacity resulting from lower integration density than a DRAM.
The miniaturization of semiconductor devices is accelerating with the recent advance in semiconductor manufacturing processes, which involves increase in distribution of basic process characteristics of the semiconductor devices. For example, in an SRAM, miniaturization of semiconductor devices involves increase in distribution of characteristics required for design such as a write margin and a sense margin. As semiconductor manufacturing processes become finer, the increased distribution leads to difficulty in development of SRAMs and reduction in stability of memory cells. As a result, yield is reduced.